Bolt with frangible, ejectable tension indicating means



May 29, 1956 c. F. BOWERSETT 2,747,454

BOLT WITH FRANGIBLE, EJECTABLE TENSION INDICATING MEANS Filed Jan. 18,1952 FIG 1.

l7 /ig" FIG 2 IO l2\ Q l3 l5 INVENTOR C. F BOWERSETT ATTORNEYS =BOLTWI'IH FRANGIBLEfEJECTKBLE IENSION FINDICATHNGIMEANS IThepresentinv'ention. relates .to apparatus .fonindicating'the amount ofstress on bolts during theetighten- .ing. thereof. .eMoreparticularlythe-invention; relates to ;apparatusi for'. indicating the .stresson.boltsz. by. providing a headedstress. rod fof-zbrittle.materiaLmounted: in.- a

-'-bore in .theabolt and .whichnwill. fracture. as .the. bolt iselongated beyond a predetermined value by..tightening. Priorart=.devices employ theprinciple .of :measuring .the amount of.torque-zrequiredto .tighten. a: bolt. LSuch devices are notentirelyasuccessful..in-.that differences the fit. fthIB8dSg burrs.on..the afaceseof abu'tting parts and other causesafiecetheeeading of.thetorque measuring device and thus such devices do not give anaccurate. measurement-of tightness, stress-or tension on the boltitself.In other devices which measure elongation of bolts; expensive measuringinstruments are em- 1.- -pl0yed .which are difiicult to maintain: inaccurate conidition andit is diflicult or impossibleto use suclr devices-in=placeswhere space is at a premium.

. .=The. device of the present invention as aforesaid is for the purposeof indicating a desired tensionnor: stress on a bolt, cap screw, or studby the breaking of an element made of a suitable plastic or other lowelasticity material, the element in breaking at a weakened portionthereof is propelled outwardly of the bore in the bolt. The breakingpoint of the material of which the element is made being known, thestress along the length of the bolt may be accurately measured.

An object of the present invention is to provide apparatus for measuringthe stress on a bolt, screw or stud and which is automatic in operation.

A further object is to provide a bolt or other threaded securingapparatus having a self-contained stress indicating device and which islow in cost.

A still further object is to provide a new and improved apparatus forindicating a desired amount of elongation on a threaded securing devicein a simple and efiicacious manner.

Other objects and many of the attendant advantages of this inventionwill be readily appreciated as the same becomes better understood byreference to the following detailed description when considered inconnection with the accompanying drawings wherein:

Fig. 1 is a sectional view of a bolt in accordance with the presentinvention;

Fig. 2 is a sectional view similar to Fig. l and showing the bolt undera desired tension;

Fig. 3 is a perspective view of the plastic insert; and

Fig. 4 is a reduced sectional view of a slightly modified bolt for usewith a depth gage or the device of Fig. 3.

Referring more particularly to the drawing wherein like numeralsindicate like parts throughout the several views, indicates generally abolt, it being understood that the present invention may be used withequal facility with studs or other forms of screw-threaded securingdevices. Bolt 10 is provided with a head 11 and a screw-threaded shank12, an axial bore 13 extending part 2,747,454 .T'Patented. May. 29,1956

.. -not been tightened, therefore. there is no-tension: between shank'12 and head 11; while FigQ Z illustrates rtheabolt ttightened and .theirangible element .15 brokenzatathe weakened -portion.17-.thereofitbeing understood.- that the bolt is elongated by.tightening thusebreaking element .1 15, the breaking-point ofelement '15vbeing governedby 4 the elastic-limit. of the materialemployedetherein-and 1 :tl1e.-size-.of theweakened portion. Itis-clear--thatwas the: bolt. -is .elongated.- shoulder -19 bears moreiorcihly .-.againsthead -18 of element.- 15 fthus stretching .-the

1 weakened portion. 17- beyond athe elastic -limit athereof and-causing.theheadeclportion 18-1to rbei-severed .-from the securedportion'of.the.-element andrbe suddenly and forcibly ejeetedtfrom the-boltdt),-Fig.-.2, as= avisual signal-tosthe operatorvthat the bolthas heen,properly -and"sufiicientlyetightened.

ItJis, also,--clearl-.-that ahigh degree-of accuracy innin- 5LdicatingJther -strain or-rtensiorr ona-bolt -.may -"beJ-provided in? aneconomical. manner by the. device. tof the ,presentvinvention, the"element-15 sbeing--adapted -to molding zthusav eliminating machiningrand v ensuring ac- =--;-curacy whenamadecin: large,- quantities. IThe-rdevice= is assembled easily withoutvcomplicated procedures ninvolving a:.high .degreezof accuracy-,r it being only neces- Zsary to.,press the---elernent 15-into bore .13 during the \eernenting-operation until head18=restsonseat r19.

By providing a frangible indicating element composed of brittle materialin which two rigid elongated portions or sections are interconnected bya weakened portion or section of uniform decreased cross sectional areaand the end portions of the elongated sections are secured to the endportions respectively of the axial bore of a bolt, the elongation ofthat portion of the bolt between the points of connection with theindicating element as the bolt is tightened is of the same order ofmagnitude as the elongation of the weakened section. Furthermore, sincethe length of the weakened section is but a fractional portion of thelength of the bolt, the strain applied to the weakened section isgreater than the strain applied to the bolt. A strain amplifyingarrangement is thus provided in which the degree of strain in theweakened section is proportional to the ratio of that portion of thelength of the bolt between the points of connection with the indicatingelement with respect to the length of the weakened section. From theforegoing it will be understood that the indicator operates on theprinciple of a strain multiplier in which the ratio of strainmultiplication may be controlled by the length of the weaken section, itbeing merely necessary that the weakened section is composed of brittlematerial suitable for the purpose such, for example, as the plasticinsert or indicator 15, Fig. 3, disclosed herein.

As shown in the modification of Fig. 4 a bolt 21, having a head 22 and athreaded shank 23, is provided with an axial bore 24 extending part waytherethrough. It is apparent that a depth gage (not shown) may beinserted to the bottom of bore 24 before the bolt is tightened in use tosecure a measurement of the distance from the bottom to the outer faceof head 22 or, if desired, the device of Fig. 3 may be employedtherewith. The bolt is tightened until a desired amount of elongation ofthe bolt, as measured by the depth gage has and used by or for theGovernment of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

What is claimed and desired to be protected by Letters Patent of theUnited States is:

1. A bolt having means for indicating a predetermined elongationresulting from the tightening thereof comprising, in combination,anelongated threaded member having a head at one end thereof and anaxial bore extending part way therethrough, said bore being reduced atthe closed end thereof and enlarged at the open end thereof to provide ashoulder within said bore, a rod composed of brittle material and havinga pair of mutually spaced cylindrical elements, one of said elementsbeing disposed within and secured to said bolt at the reduced end ofsaid bore, means disposed within the reduced end of said bore inengagement with said one cylindrical element and the bolt for securingthe rod to the bolt, a weakened portion of reduced cross sectional areaintegrally formed with said cylindrical elements and disposedtherebetween, an indicating head integrally formed with the othercylindrical element and set in the enlarged end of the bore normallyengaging said shoulder whereby the weakened portion of the rod will bebroken and said indicating head will be forcibly ejected from saidelongated bore in response to the sudden release of stress forces withinthe rod when said threaded member is elongated by tightening the boltsulficiently to efliect said predetermined elongation.

2. A bolt capable of elongation to a precise and predetermined value,comprising a body member having an axial bore extending from the headthereof to a dead end of said bore within the member, an indicatorcomposed of brittle material and arranged within saidbore comprising apair of mutually spaced elongated cylindrical portions of less diameterthan said bore, an indicat-' ing head formed integrally with one of saidcylindrical portions and having a shoulder normal to the axis of saidcylindrical portions, said shoulder normally engag ing the head of thebolt, a weakened section of less cross sectional area than saidcylindrical portions integrally formed with said cylindrical portionsand disposed therebetween such that said weakened section is fracturedand the indicating head is detached in response to a predeterminedelongation of the weakened section, and means connecting the other oneof said cylindrical portions to the bolt at the dead vend of the bore,said indicating head being detached and forcibly ejected from the boltas said weakened section is broken in response to a predeterminedelongation thereof corresponding to the elongation of the bolt as thebolt is tightened. i

3. A bolt according to claim'2 and including a second bore in said bodymember of greater diameter than said indicating head and coaxial withsaid axial bore whereby the indicating head normally engages the bottomof said second bore. I I

4. A bolt according to claim 3 in which said second bore is of a depthcorresponding substantially to the depth of said indicating head.

5. A bolt according to claim 2 in which the indicator is composed ofbrittle plastic material.

References Cited in the file of this patent UNITED STATES PATENTS1,532,736 Dodds Apr. 7,-1925 2,413,797 Stone Ian. 7, 1947 2,503,141Stone Apr. 4, 1950 2,571,718 Howes Oct. 16, 1951 2,600,029 Stone June10, 1952

